As a process for producing hydrocarbons that can be used as feedstocks for liquid fuel products such as naphtha (raw gasoline), kerosene and gas oil, a process that employs a Fischer-Tropsch synthesis reaction (hereinafter also abbreviated as “FT synthesis reaction”) which uses a synthesis gas containing mainly carbon monoxide gas (CO) and hydrogen gas (H2) as a feedstock is already known.
In terms of the synthesis reaction system used for synthesizing the hydrocarbons via the FT synthesis reaction, a bubble column slurry bed FT synthesis reaction system in which the FT synthesis reaction is conducted inside a reactor, by blowing the synthesis gas through a slurry prepared by suspending catalyst particles within liquid hydrocarbons has already been disclosed (see Patent Document 1).
In a typical FT synthesis reaction, during a gas-liquid separation step that is provided either as part of the reaction step or following the reaction step, a gas-liquid separation is performed that yields a liquid phase composed of the liquid reaction products and a gas phase containing an unreacted synthesis gas (hydrogen gas and carbon monoxide gas). This gas-liquid separation step is generally conducted at a comparatively high temperature in order to maintain the fluidity of the wax fraction contained within the reaction product, and therefore the gas phase tends to contain not only the unreacted synthesis gas, but also those light hydrocarbons among the FT synthesis reaction products that have a relatively low boiling point. On the other hand, the liquid phase is composed of a heavy hydrocarbon oil having a relatively high boiling point. The separated gas phase is then cooled, and a second gas-liquid separation is performed, yielding liquid hydrocarbons (a light hydrocarbon oil) and a gas containing mainly hydrocarbons that are gases at normal temperatures (typically hydrocarbons having a carbon number of 4 or less) and the unreacted synthesis gas.
The thus obtained light hydrocarbon oil and heavy hydrocarbon oil are stored temporarily in separate buffer tanks, and the light hydrocarbon oil and the heavy hydrocarbon oil are then discharged from the respective buffer tanks, mixed together, and then supplied, for example, to a multi-stage fractionator fitted with trays.
In the fractionator, the mixed oil containing the light hydrocarbon oil and the heavy hydrocarbon oil is fractionally distilled into, for example, a naphtha fraction that is discharged from the top of the fractionator, a middle distillate that is discharged from the central section of the fractionator, and a wax fraction that is discharged from the bottom of the fractionator. Each of these fractions passes through an upgrading step in which the fraction is subjected to hydroprocessing and fractional distillation, thus forming various liquid fuel base stocks.